Many types of springs have been developed for use in a variety of mechanical applications where particular force-displacement characteristics are required. Many of these spring designs are not suitable for applications requiring springs of small size and precise free length, however, since they cannot be manufactured to suitably precise tolerances on various physical dimensions. Small-size extension springs have been developed that consist of a single wire preformed into a coil and having hooks at either end. These extension springs can be manufactured to relatively precise tolerance on physical dimensions. However, these conventional extension spring designs have various drawbacks.
First, a conventional extension spring is prone to failure due to mechanical stresses at end hooks of the extension spring whereby the spring is attached to a spring post. Failure mode of the extension spring in such cases is generally catastrophic, causing complete failure of the spring. Second, in the conventional extension spring, the coil of the spring is subject to forces due to shock or vibration that can cause transverse deflection of the spring coil. These transverse deflections can cause the end hooks of the extension spring to disengage from their posts or can cause the spring coil to make contact and interfere with other components of a mechanical system in which the extension spring is employed.